For the purpose as mentioned above, two basic types of boiler structures have been widely used. The first of said basic types is often referred to as horizontal drum boiler system. The capacity range of such horizontal drum boilers is substantially limited by the mechanical stength characteristics. For high capacity boilers are hence boilers of the second basic type, referred to as "stud-tube wall boiler" more frequently in use.
Known horizontal drum boilers have drawbacks which in most cases outweigh the advantages. Their main mechanical and calorific disadvantages may be listed as follows:
The water space is surrounded by a double shell of substantially large dimensions, said double shell consisting of cylindrical shell rings, of dished end plates and of substantially planar discs as wall partitions.
Increasing internal overpressure and increased power capacity may only be met and maintained by using substantially thick walls. It is well known that the wall thickness needed increases linearly with internal pressure and diameter in case of cylindrical shell rings, while there is a more progressive increase in case of planar wall partitions, whereby the possibilities of the increase of power capacity are limited.
Increased wall thickness means a smaller heat transfer coefficient. Hence, the surface temperature of the heated wall surfaces will be substantially higher.
Decrease in heat transfer characteristics and increased surface temperature result in loss of life.
There is an unequal and unstable thermal load distribution on the combustion chamber surface along the axis of the jet of flame, whereby certain surface areas are overheated while others remain below optimal thermal load.
Due to the above mentioned high wall thickness, specific structure material consumption rated to boiler capacity is relatively high and thus, the utilization of material is far below optimum values that involves high investment costs and also drawbacks of technological character.
Circulation of the heat carrier is not harmonised with thermal load. There is a stratified flow of flue gas leaving the combustion chamber and entering the convective heater. Hence, the temperature of the flue gas is in certain areas of the equipment higher than permissible while in other areas said temperature lies below the allowable values which results in higher calorific losses and in increased corrosion respectively.
Drawbacks in mechanical strength emerge from the structure itself. The firing fundamentals such as the unequal thermal load of the combustion chamber are partly a consequence of the furnace installation characteristics. However, they are also dependent from the type of burner applied. Similar relations exist concerning heat dissipation too.
Improved measuring techniques developed in the past few years only, have revealed access to a more accurate determination of the energy distribution of heat radiation within the combustion chamber. As a consequence, the unequal thermal load of the heated surfaces could not be measured earlier. This is why with boilers of conventional structure a proper utilization of the heat radiation energy has not been treated with sufficient care and hence, its problem has not yet been solved in known boilers of the type in question. The now wide-spread applications of measuring methods and isntruments working in the infrared range of radiation has opened the way to a deeper analysis of heat distribution within the combustion chamber and to an industrial utilization of the results learned.
It has been discovered that boiler structures showing both optimal firing and calorific data can only be designed by applying a combustion chamber having changing, varying, non-uniform circular cross section around the axis of the jet of flame. The diameter of the cross section should harmonize with the change of the heat radiation along the flame axis. Based on the above principle, boilers having really optimal calorific and life charateristics have been designed. Experience has shown however, that drawbacks arise with the above new structures in the field of manufacture. While with these boilers, due to their varying, non-uniform cross section a substantially equal thermal load of all heated surfaces has been achieved, manufacturers applying conventional technology, equipment and tools of manufacture heavily complain that a change and a renewal of their whole technology and equipment would be far too complicated and expensive.
An object of the present invention is to provide a horizontal drum-type boiler with at least equally optimal firing and calorific characteristics as newly developed known boilers having non-uniform combustion chamber cross section show and which, at the same time, are free of the above mentioned drawbacks of said known structures.
Another more specific object of the present invention is to provide a new improved boiler structure of the horizontal drum type which has a uniform circular cross section of the combustion chamber while still a substantially equal specific thermal load of the heated surface along the axis of the jet of flame is achieved and maintained.